The "Submit contribution" button is greyed out and says I need to push a branch, which I've already done.
The branch is not visible here,
but it is visible here:
I thought... maybe I just pushed an unmodified branch:
but the button was still greyed out.
Eventually I noticed I wasn't signed in, but the experience seemed misleading. I'm not sure why I'm ever not signed in, TBH, so I get caught by surprise by stuff like this a lot. Which I guess is good because then I can create the ticket :-)